10 is the highest page rank one can get. There are several sites apart from google which has 10 PR . Visit this link
http://www.seocompany.ca/pagerank/pr-10-pages.php
Basic behind the calculation of PR is that every backlink you are getting has a vote for you. The more links at your site the better it is. But all back links are not equal. Pagerank with a high PR has a bigger vote. Plz visit this link to understand how PR is calulated.
http://www.pagerank.dk/Pagerank-form...troduction.htm
Cheers